This invention relates to a valve positioner for controlling a valve which affects a process variable and more particularly, to such valve positioners having microprocessors.
There is a desire to improve the accuracy, dynamic performance and stability with which valve positioners operate, and to provide real-time diagnostics to a control room, for quality auditing requirements and so that maintenance and plant down-time can be predicted rather than suffer emergency shutdowns or unnecessary planned valve maintenance.
Various types of positioners are used in the process control industry. Some positioners are mechanically coupled to an actuator while some incorporate the actuator within the positioner. The actuator provides means for physically positioning the valve and may be electric, hydraulic or pneumatic. Electric actuators have a current signal which drives a motor which positions the valve. Hydraulic actuators have oil-filled means for positioning the valve. By far the most common in the process control industry, a pneumatic actuator has a piston or a combination of a spring and diaphragm. Depending on the application and the level of control integration, positioners receive several types of input from a controller which are representative of the desired valve position. One type is a current input having a 4-20 mA or 10-50 mA magnitude, a second is a digital signal superimposed on the current signal and a third is a fully digital input such as Fieldbus or Modbus.RTM.. Alternatively, the positioner may receive a 3-15 pound per square inch (PSI) pneumatic input representative of the desired valve position. Depending on the level of integration and the application as well, positioners have different types of outputs. Some positioners provide an output current to a motor, while still others have a fast responding hydraulic output. The most common type of positioner output is a 0-200 PSI pneumatic output. Positioners, as the word is used in this application, includes all these field mounted instruments, including the various inputs and outputs, and their respective means for positioning valves, if applicable.
In the most common case of a spring and diaphragm actuator, the diaphragm deflects with the pressure delivered by the positioner, thereby exerting a force or torque on a control valve stem or rotary member, respectively, so as to change the position of the valve. Almost all positioners have a mechanical or an electronic position sensor to provide a position signal and some of them feed the position signal back into a microprocessor-based control section of the positioner. No matter what the specific means are for delivering force to position a valve, positioners having microprocessor based control algorithms are known. Existing positioners improve the loop dynamic response, but have a limited bandwidth so that their usage is limited to slow control loops such as one which controls level in a tank or temperature in a reactor.
One obstacle to ideal valve dynamic position control is that the valve characteristic (defined in this application as the relationship between flow and stem position or angle) deviates from the published valve characteristics by as much as five percent. Such non-ideality typifies all three major types of control valve characteristics: linear, equal percentage and quick opening. Furthermore, rotary and sliding stem valves may exhibit a nonlinear relationship between the actuator force provided to the valve and the flow through the valve, which is difficult for the inherently linear positioner to control even with the present valve stem position feedback. In fact, rotary valves have a non-monotonic torque vs. flow function as a result of the flow induced dynamic torque on the ball/butterfly in the valve. Everyday wear on valve components contributes to non-ideality in the control loop as well. In practice newly installed loops are "detuned", or purposefully assigned non-ideal control constants, to compensate for wear so that the loop remains stable over a wide variety of conditions. Compounding these issues of static and dynamic control accuracy, valve-related loop shutdowns are undesirable and expensive for industry.
The Electric Power Research Institute estimates that electric power utilities would save $400 million U.S. dollars if each control valve operated only one week longer each year. Most plants schedule regular maintenance shutdowns to monitor and repair valves, replace worn packing and worn out valve components so as to avoid even more costly and hazardous emergency shutdowns. Diagnostic systems which monitor valve integrity are known, but are generally configured to diagnose problems in valves disconnected from a process. One real-time control valve has limited diagnostics capability.
A positioner, control valve and actuator are assembled and properly configured for installation in a time consuming process called bench-setting. During benchset, an operator manually sets the valve's maximum travel position (called the stroke position), the minimum travel position (called the zero), limit stops and stiffness parameters. The process is iterative because the settings are not independent.
Thus, there is a need for a microprocessor-based valve positioner easily configurable at benchset, with increased bandwidth and improved dynamic positioning accuracy, which also has real-time diagnostics to provide valve and actuator integrity information.